1. Field of the Invention
The invention relates generally to audio amplification systems, and more particularly to systems and methods for automatically adjusting the alignment of high-side and low-side pulse width modulated signals to improve dead time and shoot-through conditions.
2. Related Art
Pulse Width Modulation (PWM) or Class D signal amplification technology has existed for a number of years. PWM technology has become more popular with the proliferation of Switched Mode Power Supplies (SMPS). Since this technology emerged, there has been an increased interest in applying PWM techniques in signal amplification applications as a result of the significant efficiency improvement that can be realized through the use of Class D power output topology instead of the legacy (linear Class AB) power output topology.
Early attempts to develop signal amplification applications utilized the same approach to amplification that was being used in the early SMPS. More particularly, these attempts utilized analog modulation schemes that resulted in low performance applications. These applications were complex and costly to implement. Consequently, these solutions were not widely accepted. Prior art analog implementations of Class D technology have therefore been unable to displace legacy Class AB amplifiers in mainstream amplifier applications.
Recently, digital PWM modulation schemes have surfaced. These schemes use Sigma-Delta modulation techniques to generate the PWM signals used in the newer digital Class D implementations. These digital PWM schemes, however, did little to offset the major barriers to integration of PWM modulators into the total amplifier solution. Class D technology has therefore continued to be unable to displace legacy Class AB amplifiers in mainstream applications.
One of the problems with prior art systems and methods is that they do not provide a mechanism for automatically adjusting the timing of signals provided to the output stage of a PWM amplifier. A typical PWM amplifier includes a PCM-to-PWM modulator that receives a PCM audio signal and generates a pair of PWM signals. The signals are provided to a driver, which drives each of the signals to a corresponding FET (field effect transistor) in the output stage. One of the PWM signals is a high-side signal that drives a high-side FET, and the other signal is a low-side signal that drives a low-side FET. Each of the PWM signals turns the corresponding FET on and off. Generally speaking, the signals are intended to turn the FETs on and off alternately. In other words, when the high-side FET is turned on, the low-side FET should be turned off, and vice versa. (It should be noted that, while the present embodiment uses FETs, other embodiments may use other types of transistors.)
The FETs, however, do not turn on and off instantaneously, but instead require a certain amount of time to change between the “on” state and the “off” state. There may therefore be some overlap between the times each of the FETs is turned on. If there is little overlap, a “dead time” is introduced during which the signal produced by the output stage does not follow the input signal—and thus distortion is created. If there is a substantial amount of overlap, there will generally be less distortion, but the output stage may draw a great deal of current because both FETs are turned on and the current is allowed to flow essentially directly from a voltage source to ground. This current is referred to as “shoot-through” current. It is desirable to be able to adjust the amount of overlap between the signals in order to control the balance of the dead time and shoot-through current. This is particularly true since there are a number of factors that cause variations in the delays incurred by each of the signals, including component variations, environmental conditions (e.g., temperature), etc.
In conventional PWM amplifiers, the relative timing of the high-side and low-side signals is adjusted by means of analog components, such as the resistors and capacitors in an RC circuit. These components are not repeatedly adjusted, but are instead adjusted a single time (e.g., in the lab). After this one-time adjustment, the timing is not changed any more. As a result, even if the initial adjustment were exactly optimal, factors that change during the life of the product (e.g., aging of the components, changes in temperature during operation, etc.) may cause the timing itself to change. Now one-time adjustment may therefore no longer achieve the desired balance of dead time and shoot-through current.
It would therefore be desirable to provide a mechanism for automatically adjusting the relative timing of the high-side and low-side channels to achieve optimal timing for the current conditions.